1. Field of the Invention
The invention relates to an exhaust gas purification system and an exhaust gas purification method for an internal combustion engine having an adsorption-reduction type NOx catalyst provided in the exhaust passage.
2. Description of the Related Art
An exhaust gas purification system for an internal combustions engine has an adsorption-reduction type NOx catalyst (will be simply referred to as “NOx catalyst” where appropriate) provided in an exhaust passage. NOx catalysts adsorb NOx in an exhaust gas under an oxidizing atmosphere and reduce the adsorbed NOx under a reducing atmosphere.
Such NOx catalysts, however, adsorb SOx as well as NOx. Therefore, in an exhaust gas purification system for an internal combustion engine having a NOx catalyst, so-called SOx poisoning recovery control is executed to reduce the SOx adsorbed in the NOx catalyst During the SOx poisoning control, the exhaust gas air-fuel ratio of the exhaust gas entering the NOx catalyst, which is the ratio between the air contained in the exhaust gas entering the NOx catalyst and the fuel element contained in the same exhaust gas and acting as a reducing agent at the NOx catalyst, is reduced to a target exhaust gas air-fuel ratio enabling SOx reduction reactions, and the temperature of the NOx catalyst is increased to a target temperature enabling SOx reduction reactions.
Japanese Patent Application Publication. No. 2002-155724 (JP-A-2002-155724) describes a technology in which the exhaust gas air-fuel ratio is controlled by controlling the amount of fuel injected by a post-injection and/or controlling the amount of fuel added from a fuel addition valve provided upstream of the NOx catalyst. Also, Japanese Patent Application Publication No. 2001-280125 (JP-A-2001-280125) and Japanese Patent Application Publication No. 2003-120373 (JP-A-2003-120373) have been also studied as references. However, none of these related technologies specifically addresses the necessity for minimizing the reduction of the fuel economy that may be caused by the SOx poisoning recovery control.